The primary goal of this project is to identify the biochemical steps involved in phototransduction. Our approach is to use pharmacological agents to modify the intracellular and extracellular millieu of the photoreceptor in order to modify phototransduction. We have also developed a method using microspectrophotometry for monitoring intracellular pH and Ca while simultaneously monitoring sensitivity. Our pharmacological studies suggest that a guanyl nucleotide binding protein is directly involved in the biochemical pathway of phototransduction.